The modern railway consists of steel rails secured on top of sleepers or cross-ties. Cross-ties or ties are often constructed from wood or concrete. When concrete ties are used on a railway it is common practice to use a multi-part plastic or plastic composite tie pad between the base of the steel rail and the concrete tie. The intended function of this tie pad is to mitigate the impact forces of rail traffic to prevent damage to the concrete tie. An inward angle is engineered into the concrete tie and is referred to as the “cant”. The purpose of the cant is to angle the rails to counteract the load moment created on the tie by the train load, which is transmitted through the rails into the tie. The cant is usually cast into the geometry of concrete ties.
Over time the surface of the concrete beneath the tie pad wears out and the concrete can abrade. This wear, termed rail seat abrasion, is a common occurrence for railroads that handle heavy tonnage freight and in geographic areas prone to higher levels of moisture. Rail seat abrasion can occur in other areas of high track loadings such as in curved track or where steep grades are present. Severe rail seat abrasion can compromise the effectiveness of fasteners used to secure the steel rails to the concrete cross tie. The diminished effectiveness of the rail fasteners leads to an unsafe condition, possibly resulting in train derailment.
Modern day concrete tie repair techniques and preventive maintenance involve removing the steel rail and tie pads then applying an epoxy or other polymer material to build up the worn area. These materials are weather sensitive and the cure times for them to harden enough to allow the rail to be put back on can be from 5 to 30 minutes or longer. Once the polymer has cured, a new tie pad is placed on the tie seat (rail seat) area of the concrete tie, then the steel rail is replaced. It has become a standard for all class one railroads to repair tie seat abrasion when replacing a worn (steel) rail, which is usually done one quarter mile at a time. While maintenance work is being done to the rail the railway must be taken out of service, resulting in train traffic scheduling issues and train delays. The time spent by maintenance crews to repair the ties results in huge train delays and lost revenue from the delays. Most railroads today operate as scheduled railroads which means trains need to get from point A to point B in a predetermined time. Delays for maintenance procedures like this however, necessarily cause problems for the transportation departments of the railroads.